Metallic colloidal solution and inkjet-use metallic ink

ABSTRACT

A metallic colloidal solution (a) includes a water-based dispersion medium that is easy in handling with regard to safety and environment and metallic particles having a uniform particle diameter and being excellent in properties such as conductivity and (b) has properties suitable for various printing methods and ink-applying methods. In addition, an inkjet-use metallic ink incorporating the metallic colloidal solution has properties suitable for the inkjet printing method. The metallic particles are deposited by reducing metallic ions in water and have a primary-particle diameter of at most 200 nm. The dispersion medium is made of a mixed solvent of water and a water-soluble organic solvent. The metallic particles are dispersed in the dispersion medium under the presence of a dispersant having a molecular weight of 200 to 30,000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metallic colloidal solution and aninkjet-use metallic ink both suitable for forming a fine wiring circuit,a thin conductive film with a uniform thickness, and the like.

2. Description of the Background Art

Wiring circuits, conductive films, and the like are formed by usingextremely minute metallic particles having a diameter of severalnanometers to tens of nanometers or so. More specifically, such metallicparticles are dispersed in a dispersion medium under the presence of adispersant for preventing aggregation and increasing the dispersibilityto obtain a metallic colloidal solution. The metallic colloidal solutionis used as an ink for various printing methods and ink-applying methods.The ink is printed or applied on a substrate and, as required, is bakedto form a wiring circuit or a conductive film.

As the metallic colloidal solution, a water-based solution using wateras the dispersion medium has been widely used. In addition, thesolution's properties such as viscosity, surface tension, and vaporpressure (a boiling point) are limited within a narrow range. However,wiring circuits and conductive films have been formed in recent yearsthrough various printing methods and ink-applying methods, such as thespin coating method, the screen printing method, and the dispenserapplication method. Consequently, the metallic colloidal solution isrequired to have properties suitable for individual printing methods andink-applying methods.

Furthermore, as the forming method of a wiring circuit and a conductivefilm, the inkjet printing method, which uses an inkjet printer, has beenattracting attention in recent years. Studies are being made to use themetallic colloidal solution as the metallic ink for the inkjet printingmethod. However, the conventional metallic colloidal solution is usuallyproduced as a water-based solution using water as the dispersion medium.In addition, the solution's properties such as viscosity, surfacetension, and vapor pressure (a boiling point) are limited within anarrow range.

In view of the above-described circumstances, studies are beingconducted to use not only water but also various organic solvents as thedispersion medium, which directly affects the properties of the metalliccolloidal solution. For example, the published Japanese patentapplication Tokukaihei 11-80647 has disclosed the below describedmethods of producing a metallic colloidal solution (see claims 11 and 12and sections 0042, 0045, and 0046 of this application). This applicationutilizes the liquid-phase reduction method, which is known as a methodcapable of producing metallic particles with a uniform particlediameter. More specifically, Tokukaihei 11-80647 applies a method ofproducing a water-based metallic colloidal solution by depositingmetallic particles through the reduction of metallic ions in water underthe presence of a dispersant. Tokukaihei 11-80647 has disclosed thefollowing methods:

-   (a) a method of producing a metallic colloidal solution whose    dispersion medium is an organic solvent. In this method, first, a    metallic compound, which is the source of metallic ions, is solved    in an organic solvent. After the addition of a dispersant, the    metallic ions are reduced to deposit metallic particles.-   (b) a method of producing a metallic colloidal solution whose    dispersion medium is a mixed solvent of water and a water-soluble    organic solvent. In this method, first, a metallic compound, which    is the source of metallic ions, is solved in water. After the    addition of a water-soluble organic solvent and a dispersant, the    metallic ions are reduced to deposit metallic particles.

Another published Japanese patent application, Tokukai 2001-35255, hasdisclosed a method of producing a metallic colloidal solution in whichmetallic particles are dispersed in an organic solvent (in Tokukai2001-35255, the solution is called a liquid in which extremely finesilver particles are dispersed independently) (see section 0006 ofTokukai 2001-35255). This application utilizes a method of producingmetallic particles through the vapor-phase growth method. First,metallic particles growing in a vapor phase are made contact with avapor of a high-boiling-point organic solvent such as mineral spirits.Then, the metallic particles are cooled and recovered to produce thesolution.

However, in one of the methods disclosed in Tokukaihei 11-80647, whichone uses an organic solvent as the dispersion medium, the types ofmetallic compound and reducing agent both having good solubility in aspecific organic solvent are limited. This limitation poses a problem oflimiting the type of metallic particles to be formed.

On the other hand, in the other method, which uses a mixed solvent ofwater and a water-soluble organic solvent, although commonly usedwater-soluble metallic compounds and water-soluble reducing agents canbe used, many of them have low solubility in a water-soluble organicsolvent. As a result, at the time the water-soluble organic solvent isadded, some of them are deposited or forced to behave adversely to causethe reaction system to become prone to produce nonuniformity inconcentration.

This nonuniformity in concentration causes the formed metallic particlesto have variations in particle diameter. Consequently, the producedwiring circuit and conductive film may become nonuniform in propertiessuch as structure and conductivity.

When this metallic colloidal solution is used as an inkjet-use metallicink, the nozzle and another small opening of an inkjet printer tend tobe clogged. Furthermore, the formed wiring circuit and conductive filmmay become nonuniform in properties such as structure and conductivity.In addition, unreacted metallic compounds may enter the metallicparticles as an impurity. When this occurs, the metallic particles and,consequently, the wiring circuit and conductive film may be impaired inproperties such as conductivity.

On the other hand, the method disclosed in Tokukai 2001-35255 is limitedto the application that uses a high-boiling-point organic solvent, whichrequires careful handling with regard to safety and environment. Inother words, this method has a drawback of a narrow range ofapplication. Furthermore, when this metallic colloidal solution is usedas an inkjet-use metallic ink, the organic solvent such as mineralspirits may dissolve the adhesive used in the head and other componentsof the inkjet printer.

SUMMARY OF THE INVENTION

The present invention offers a metallic colloidal solution (a) thatincludes a water-based dispersion medium that is easy in handling withregard to safety and environment and metallic particles having a uniformparticle diameter and being excellent in properties such as conductivityand (b) that has properties suitable for various printing methods andink-applying methods. The present invention also offers an inkjet-usemetallic ink that incorporates the above-described metallic colloidalsolution and that has properties suitable for the inkjet printingmethod.

The present invention offers a metallic colloidal solution thatincludes:

-   (a) metallic particles that are deposited by reducing metallic ions    in water and that have a primary-particle diameter of at most 200    nm;-   (b) a dispersant having a molecular weight of 200 to 30,000; and-   (c) a dispersion medium made of a mixed solvent of water and a    water-soluble organic solvent.

A metallic colloidal solution of the present invention incorporatesmetallic particles that are produced by an ordinary liquid-phasereduction method using water and that have a uniform particle diameter.Therefore, the metallic colloidal solution can prevent variations in thestructure, conductivity, and so on of the formed wiring circuit andconductive film. In other words, the formed wiring circuit andconductive film can have increased uniformity in those properties. Inaddition, the metallic particles are nearly free from impurities such asunreacted metallic compounds. Consequently, the metallic particles havenotably high conductivity, so that they can improve the conductivity ofthe wiring circuit and conductive film.

The metallic colloidal solution incorporates a dispersion medium made ofa mixed solvent of water and a water-soluble organic solvent. Therefore,it is easy to handle with regard to safety and environment. Furthermore,properties such as viscosity, surface tension, and vapor pressure of themetallic colloidal solution can be easily adjusted to fall within arange suitable for various printing methods and ink-applying methods byadjusting the composition ratio of the water and the water-solubleorganic solvent in the mixed solvent and by selecting the type of thewater-soluble organic solvent.

A metallic colloidal solution of the present invention may be produced:

-   (a) by using as a starting material a water-based metallic colloidal    solution obtained by depositing metallic particles through the    reduction of metallic ions in water; and-   (b) without undergoing a process that completely separates the    metallic particles from water.

In this case, a water-based metallic colloidal solution obtained by thereductive deposition method is used as a starting material. Then, themetallic colloidal solution is produced without undergoing a processthat completely separates the metallic particles from water. Therefore,the metallic colloidal solution can be produced without the productionof secondary particles due to aggregation resulting from the completeseparation of the metallic particles from water. Consequently, thisprocess can prevent drawbacks caused by the secondary particles, such asa variation in particle diameter and an increase in particle diameter asa whole. As a result, the produced metallic colloidal solution maintainsa state in which the metallic particles are dispersed uniformly in thedispersion medium as nearly the primary particles, which state iscreated directly after the production of the metallic particles by thereductive deposition method. The use of such a metallic colloidalsolution can further increase the uniformity of the structure andconductivity of the wiring circuit and conductive film.

A metallic colloidal solution of the present invention may incorporatemetallic particles that are particles made of a metal selected from thegroup consisting of nickel, copper, silver, gold, platinum, palladium,and an alloy of these metals.

In this case, the use of highly conductive metallic particles made ofnickel, copper, silver, gold, platinum, palladium, or an alloy of thesemetals can further increase the conductivity of the wiring circuit andconductive film.

A metallic colloidal solution of the present invention may containmetallic particles with a content of 0.1 to 90 wt. %.

In this case, the containing of the metallic particles with any contentin the range of 0.1 to 90 wt. % enables easy adjustment of properties ofthe metallic colloidal solution so that they can fall within a rangesuitable for various printing methods and ink-applying methods.

A metallic colloidal solution of the present invention may incorporate adispersant made of an organic compound that does not contain any ofsulfur, phosphorus, boron, and a halogen atom.

In this case, the use of a dispersant made of an organic compound thatdoes not contain sulfur, phosphorus, boron, or a halogen atom canprevent not only the wiring circuit and conductive film but also othermembers such as electronic components placed in the vicinity of themfrom deteriorating because of the presence of these elements.

A metallic colloidal solution of the present invention may contain adispersant with a content of 2 to 30 weight parts per 100 weight partsof the metallic particles.

In this case, the containing of the dispersant with a content of 2 to 30weight parts per 100 weight parts of the metallic particles can preventthe metallic particles from aggregating in the metallic colloidalsolution. As a result, a highly conductive wiring circuit and conductivefilm can be formed.

A metallic colloidal solution of the present invention may incorporate awater-soluble organic solvent made of at least one material selectedfrom the group consisting of alcohol, ketone, glycol ether, and awater-soluble nitrogen-containing organic compound.

In this case, the use of the water-soluble organic solvent made of atleast one material selected from the group consisting of alcohol,ketone, glycol ether, and a water-soluble nitrogen-containing organiccompound enables easy adjustment of properties of the metallic colloidalsolution so that they can fall within a range suitable for variousprinting methods and ink-applying methods.

According to an aspect of the present invention, the present inventionoffers an inkjet-use metallic ink incorporating a metallic colloidalsolution that includes:

-   (a) metallic particles that are deposited by reducing metallic ions    in water and that have a primary-particle diameter of at most 200    nm;-   (b) a dispersant having a molecular weight of 200 to 30,000; and-   (c) a dispersion medium made of a mixed solvent of water and a    water-soluble organic solvent.

An inkjet-use metallic ink of the present invention incorporatesmetallic particles that are produced by an ordinary liquid-phasereduction method using water and that have a uniform particle diameter.Therefore, the inkjet-use metallic ink can prevent the nozzle andanother small opening of an inkjet printer from clogging. Furthermore,it can prevent variations in the structure, conductivity, and so on ofthe formed wiring circuit and conductive film. In other words, theformed wiring circuit and conductive film can have increased uniformityin those properties. In addition, the metallic particles are nearly freefrom impurities such as unreacted metallic compounds. Consequently, themetallic particles have notably high conductivity, so that they canimprove the conductivity of the wiring circuit and conductive film.

The inkjet-use metallic ink incorporates a dispersion medium made of amixed solvent of water and a water-soluble organic solvent. Therefore,it is easy to handle with regard to safety and environment. Furthermore,properties such as viscosity, surface tension, and vapor pressure of theinkjet-use metallic ink can be easily adjusted to fall within a rangesuitable for the inkjet printing method by adjusting the compositionratio of the water and the water-soluble organic solvent in the mixedsolvent and by selecting the type of the water-soluble organic solvent.

An inkjet-use metallic ink of the present invention may have a surfacetension of 20 to 60 mN/m at 25° C. and a viscosity of 0.5 to 40 mPa·s at25° C.

In this case, because the surface tension is adjusted to fall within therange of 20 to 60 mN/m at 25° C. and the viscosity of 0.5 to 40 mPa·s at25° C., the ink can be ejected satisfactorily from the nozzle by theinkjet printing method without developing any faulty ejection such asthe clogging at the nozzle and another small opening. Thus, the wiringcircuit and conductive film can be formed satisfactorily.

An inkjet-use metallic ink of the present invention may incorporate ametallic colloidal solution that is produced:

-   (a) by using as a starting material a water-based metallic colloidal    solution obtained by depositing metallic particles through the    reduction of metallic ions in water; and-   (b) without undergoing a process that completely separates the    metallic particles from water.

In this case, a water-based metallic colloidal solution obtained by thereductive deposition method is used as a starting material. Then, theinkjet-use metallic ink is produced without undergoing a process thatcompletely separates the metallic particles from water. Therefore, theinkjet-use metallic ink can be produced without the production ofsecondary particles due to aggregation resulting from the completeseparation of the metallic particles from water. Consequently, thisprocess can prevent drawbacks caused by the secondary particles, such asa variation in particle diameter and an increase in particle diameter asa whole. As a result, the produced inkjet-use metallic ink maintains astate in which the metallic particles are dispersed uniformly in thedispersion medium as nearly the primary particles, which state iscreated directly after the production of the metallic particles by thereductive deposition method. The use of such an inkjet-use metallic inkcan further reliably prevent the clogging at the nozzle and anothersmall opening of the inkjet printer. In addition, the uniformity of thestructure and conductivity of the wiring circuit and conductive film canbe increased.

An inkjet-use metallic ink of the present invention may incorporate ametallic colloidal solution incorporating a water-soluble organicsolvent made of at least one material selected from the group consistingof alcohol, ketone, glycol ether, and a water-solublenitrogen-containing organic compound.

In this case, the use of the water-soluble organic solvent made of atleast one material selected from the group consisting of alcohol,ketone, glycol ether, and a water-soluble nitrogen-containing organiccompound enables easy adjustment of properties of the inkjet-usemetallic ink so that they can fall within a range suitable for theinkjet printing method.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a photograph taken with an electron microscope, the photographshowing the structure of the silver particles in a film formed using thesilver colloidal solution of Example 1 of the present invention.

FIG. 2 is a photograph taken with an electron microscope, the photographshowing the structure of the silver particles in the printed figureformed by using the inkjet-use silver ink of Example 11 of the presentinvention, the photograph being taken before the baking of the printedfigure.

FIG. 3 is a photograph taken with an electron microscope, the photographshowing the structure of the film forming a wiring circuit formed bybaking the printed figure shown in FIG. 2, the film being formed by theconsolidation of molten silver particles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is explained below. A metallic colloidal solutionand an inkjet-use metallic ink of the present invention include (a)metallic particles that are deposited by reducing metallic ions in waterand that have a primary-particle diameter of at most 200 nm, (b) adispersant that has a molecular weight of 200 to 30,000, and (c) adispersion medium made of a mixed solvent of water and a water-solubleorganic solvent.

In the above description, the reason why the primary-particle diameterof the metallic particles is limited to at most 200 nm is explainedbelow. Large metallic particles having a primary-particle diameter ofmore than 200 nm have poor dispersibility in a metallic colloidalsolution. They tend to aggregate to produce secondary particles. Evenwhen they do not aggregate, they reduce the fluidity of the metalliccolloidal solution. Therefore, a metallic colloidal solution includingsuch large metallic particles cannot have properties required for an inkto be used for various printing methods and ink-applying methods.Furthermore, when it is used as an inkjet-use metallic ink, the nozzleand another small opening tend to be clogged. In addition, the formedwiring circuit and conductive film become nonuniform in structure andconductivity due to the production of the secondary particles and otheradverse causes.

On the other hand, extremely fine metallic particles having aprimary-particle diameter of at most 200 nm have superior dispersibilityin a metallic colloidal solution. They have no tendency to aggregate.They have increased fluidity. Consequently, as described earlier, ametallic colloidal solution of the present invention including metallicparticles that have a primary-particle diameter of at most 200 nm cansatisfy the requirement to have the most suitable range of propertiessuch as viscosity, surface tension, and vapor pressure by adjusting thecomposition ratio of water and a water-soluble organic solvent and byselecting the type of the water-soluble organic solvent. Similarly, aninkjet-use metallic ink of the present invention can satisfy therequirement to have the most suitable range of properties for the inkjetprinting method. In addition, it has no tendency to clog the nozzle orto produce other defective conditions. Furthermore, the wiring circuitand conductive film formed by using a metallic colloidal solution and aninkjet-use metallic ink of the present invention have extremely uniformstructure and conductivity.

The lowest value of the primary-particle diameter of the metallicparticles has no particular limitation. Theoretically, it is possible touse the particles having the minimum diameter that can have conductivityas a metal. Practically, however, it is desirable that theprimary-particle diameter be at least 1 nm. In other words, it isdesirable that the metallic particles have a primary-particle diameterof 1 to 200 nm.

It is desirable that the content of the metallic particles both be 0.1to 90 wt. % of the total weight of the metallic colloidal solution andbe 0.1 to 90 wt. % of the total weight of the inkjet-use metallic ink.If the content is less than 0.1 wt. %, the content is excessively small,so that it may be impossible to form a wiring circuit and a conductivefilm having sufficient thickness and conductivity by any of the printingmethods and ink-applying methods. Conversely, if the content is morethan 90 wt. %, the fluidity decreases excessively, so that it may beimpossible to obtain a metallic colloidal solution and an inkjet-usemetallic ink suitable for various printing methods and ink-applyingmethods.

It is desirable that the primary-particle diameter and the percentagecontent of the metallic particles be determined to fall in a range mostsuitable for an individual printing method and ink-applying method byselecting from the above-described range so that the most suitableproperties can be achieved when a specific printing method orink-applying method is adopted for forming a wiring circuit and aconductive film. The metallic particles may be composed of a metalselected from various metals and alloys. In particular, in view ofgiving good conductivity to the wiring circuit and conductive film, itis desirable that the metallic particles be composed of nickel, copper,silver, gold, platinum, palladium, or an alloy of these metals.

As the dispersant, a dispersant that has a molecular weight of 200 to30,000 is selected from various dispersants having good solubility inwater or an water-soluble organic solvent. If the molecular weight ofthe dispersant is less than 200, the effect of stably dispersing themetallic particles cannot be obtained.

If the molecular weight is more than 30,000, in this case also, theeffect of stably dispersing the metallic particles cannot be obtained.Furthermore, the dispersant having such a high molecular weight mayintervene between metallic particles in the formed wiring circuit andconductive film to decrease the conductivity.

On the other hand, the dispersant having a molecular weight of 200 to30,000 tends to have a structure known as the loop-train-tail structure.Consequently, it has an excellent effect of improving the dispersibilityof the metallic particles. It has no possibility of decreasing theconductivity by intervening between metallic particles. In view of thestable dispersion of the metallic particles, it is particularlydesirable that the above-described range of the molecular weight of thedispersant be narrowed to a range of 2,000 to 30,000.

Considering the prevention of the deterioration of not only the wiringcircuit and conductive film but also other members such as electroniccomponents placed in the vicinity of them, it is desirable that thedispersant be composed of an organic compound that does not containsulfur, phosphorus, boron, or a halogen atom. The types of suitabledispersant that satisfies these requirements include an amine-basedpolymer dispersant such as polyethylene imine and polyvinyl pyrrolidone,a hydrocarbon-based polymer dispersant having a carboxylic acid group inthe molecule such as polyacrylic acid and carboxymethyl cellulose, andpoval (polyvinyl alcohol). It is also desirable to use a polymerdispersant such as a copolymer having a polyethylene imine portion and apolyethylene oxide portion in one molecule (hereinafter referred to as aPEI-PO copolymer). The polymer dispersant can act as aviscosity-adjusting agent.

It is desirable that the content of the dispersant be 2 to 30 weightparts per 100 weight parts of the metallic particles. If the content isless than 2 weight parts, it may be impossible to sufficiently obtainthe dispersant-adding effect of uniformly dispersing the metallicparticles in the metallic colloidal solution and the inkjet-use metallicink. Conversely, if the content is more than 30 weight parts, theviscosity becomes excessively high. Consequently, it may be impossibleto obtain (a) an metallic colloidal solution suitable for variousprinting methods and ink-applying methods and (b) an inkjet-use metallicink having properties suitable for the inkjet printing method. Inaddition, the excess dispersant may intervene between metallic particlesin the formed wiring circuit and conductive film to decrease theconductivity.

It is desirable that the molecular weight and the percentage content ofthe dispersant be determined to fall in a range most suitable for anindividual printing method and ink-applying method by selecting from theabove-described range so that the most suitable properties can beachieved when a specific printing method or ink-applying method isadopted for forming a wiring circuit and a conductive film. It isdesirable that the type of the dispersant to be used be determined byselecting from the various dispersants so that the most suitableproperties can be achieved when a specific printing method orink-applying method is adopted for forming a wiring circuit and aconductive film.

As the water-soluble organic solvent, any organic solvent may be usedproviding that it is soluble in water and has a dielectric constant ofat least 3 at 20° C. The types of the water-soluble organic solventinclude alcohol such as methanol, ethanol, n-propanol, 2-propanol,t-butylalcohol, glycerin, dipropylene glycol, ethylene glycol, andpolyethylene glycol; ketone such as acetone and methyl ethyl ketone;glycol ether such as ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol monobutyl ether, diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, diethylene glycolmonobutyl ether, diethylene glycol dimethyl ether, diethylene glycoldiethyl ether, triethylene glycol monobutyl ether, dipropylene glycolmonomethyl ether, dipropylene glycol monoethyl ether, and tripropyleneglycol monomethyl ether; a water-soluble nitrogen-containing organiccompound such as 2-pyrrolidone and N-methyl pyrrolidone; and ethylacetate. The water-soluble organic solvent can be used either singly inone type or in combination of two or more types.

In the present invention, the dispersant is determined so that themetallic colloidal solution and the inkjet-use metallic ink can have themost suitable properties for an individual printing method andink-applying method when they are used to form a wiring circuit and aconductive film. More specifically, the dispersant is made of water anda water-soluble organic solvent, and the composition ratio of them andthe type of the water-soluble organic solvent are selected properly.When two or more types of water-soluble organic solvents are combined,the combination is selected properly.

For example, when a conductive film is formed by using the spin coatingmethod, the metallic colloidal solution is required to have lowviscosity. Conversely, when a wiring circuit is produced by using thescreen printing method or dispenser application method, the metalliccolloidal solution is required to have high viscosity. In the screenprinting method, the metallic colloidal solution is required to havehigh vapor pressure in order to retard the drying.

To be suitable for the piezoelectric-type inkjet printing method, it isdesirable that the inkjet-use metallic ink have a surface tension of 20to 60 mN/m at 25° C. and a viscosity of 0.5 to 40 mPa·s at 25° C. inorder to improve the ink-ejecting performance of the nozzle, asexplained earlier. In addition, the ink is required to have low vaporpressure to prevent a rapid increase in viscosity and rapid drying. Itis desirable that the ink have a boiling point of at least 100° C., moredesirably 120 to 300° C. In satisfying these property requirements, itis desirable that the water-soluble organic solvent be composed of acompound having relatively high molecular weight, such as glycol ether.

On the other hand, to be suitable for the thermal-type inkjet printingmethod, it is desirable that the inkjet-use metallic ink not only have asurface tension of 20 to 60 mN/m at 25° C. and a viscosity of 0.5 to 40mPa·s at 25° C. in order to improve the ink-ejecting performance of thenozzle, as with the above-described case, but also have high vaporpressure to facilitate the generation of bubbles by the heat from theheating element. It is desirable that the ink have a boiling point of atmost 150° C., more desirably 80 to 130° C. In satisfying these propertyrequirements, it is desirable that the water-soluble organic solvent becomposed of a compound having relatively low molecular weight, such aslower alcohol.

To satisfy the above-described property requirements, the compositionratio of the water and water-soluble organic solvent and the type of thewater-soluble organic solvent are selected properly. In addition, whentwo or more types of water-soluble organic solvents are combined, thecombination is selected properly. At the same time, as described above,the following items are selected: (a) the primary-particle diameter andpercentage content of the metallic particles, (b) the molecular weightand percentage content of the dispersant, and (c) the type of thedispersant.

It is desirable that a metallic colloidal solution and an inkjet-usemetallic ink of the present invention be produced by the followingprocess. The starting material is a water-based metallic colloidalsolution obtained by depositing metallic particles through the reductionof metallic ions in water. The final solution is obtained withoutundergoing a process that completely separates the metallic particlesfrom water. More specifically, first, to achieve a specificconcentration, the water-based metallic colloidal solution isconcentrated by using a rotary evaporator, by applying heat, or byremoving a supernatant fluid with the centrifugal separation method, forexample. Then, a specific amount of water-soluble organic solvent isadded to the solution to produce a metallic colloidal solution and aninkjet-use metallic ink of the present invention.

As the starting material, the water-based metallic colloidal solutioncan be produced through a conventional method. For example, awater-soluble metallic compound, which is the source of metallic ions,and a dispersant are dissolved in water. At the same time, a reducingagent is added to the water. The metallic ions are subjected toreduction reaction for a specific time desirably under the conditionthat the solution is stirred. Thus, the water-based metallic colloidalsolution is produced.

The water-soluble metallic compound for supplying metallic ions may beselected as follows, although it is not limited to the followingmaterials. In the case of silver, its types include silver nitrate (I)(AgNO₃) and silver methane-sulfonide (CH₃SO₃Ag). In particular, silvernitrate (I) is desirable. In the case of gold, its types includetetrachloroaurate (III) 4hydrate (HAuCl₄.4H₂O). In the case of platinum,its types include dinitrodiamine platinate (II) (Pt(NO₂)₂(NH₃)₂) andhexachloroplatinate (IV) 6hydrate (H₂[PtCl₆]-6H₂O). In addition, theabove-described water-soluble metallic compounds may be used after theyare transformed into a complex by using ammonia, citric acid, or thelike, as required.

As the reducing agent, various water-soluble reducing agents may beused. Nevertheless, it is desirable that the reducing agent have thesmallest possible particle diameter. In addition, in view of theformation of uniform metallic particles, it is desirable to use areducing agent having a weak reducing power.

The types of the reducing agent that satisfies the foregoingrequirements include alcohol such as methanol, ethanol, and 2-propanol;ascorbic acid; ethylene glycol; glutathione; organic acids such ascitric acid, apple acid, and tartaric acid; reducing saccharides such asglucose, galactose, mannose, fructose, sucrose, maltose, raffinose, andstachyose; and sugar alcohols such as solbitol.

The primary-particle diameter of the metallic particles can be adjustedso as to fall within the above-described range by adjusting not only thetype and percentage content of the metallic compound, dispersant, andreducing agent but also the stirring rate, temperature, and time at thetime the metallic compound undergoes the reduction reaction.

A metallic colloidal solution of the present invention can be usedsuitably as the ink for various printing methods and ink-applyingmethods by properly adjusting its properties. As described above, thevarious methods include the spin coating method for forming a conductivefilm and the screen printing method and dispenser application method forforming a wiring circuit. An inkjet-use metallic ink of the presentinvention can be used suitably for various inkjet printing methods byproperly adjusting its properties. As described above, the variousmethods include the piezoelectric- and thermal-type inkjet printingmethods for forming a wiring circuit and a conductive film.

EXAMPLE

The present invention is explained below based on Examples andComparative examples.

(Silver Colloidal Solution)

Example 1

A silver nitrate-ammonia solution was prepared by, first, dissolving 24grams of silver nitrate in 150 grams of pure water and, then, addingammonia water to the liquid to adjust its pH to 11.0. Next, 12 grams ofpolyvinyl pyrrolidone (molecular weight: 30,000) as the dispersant wasadded to the silver nitrate-ammonia solution and dissolved in it. Then,100 grams of ethylene glycol as the reducing agent was added to thesolution. The solution was stirred at a stirring rate of 1,000 rpm for180 minutes at 40° C. to cause the chemical reaction to proceed. Thus, awater-based silver colloidal solution having a yellow plasmon absorptionwas obtained.

Next, the obtained silver colloidal solution was centrifuged at 20,000 Gfor 20 minutes to remove impurities lighter than the silver particles.This removing operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using a particle size analyzer that applied the laser Dopplermethod (the analyzer: made by Nikkiso Co., Ltd. based in Japan, thebrand name: Microtrac UPA150EX). The result revealed that there was asharp peak at a position of 5 nm.

The obtained silver colloidal solution was concentrated with a rotaryevaporator to reduce the water content to 20 wt. %. Acetone as thewater-soluble organic solvent was added to the solution. Thus, a silvercolloidal solution whose dispersion medium was a mixed solution of waterand acetone was produced.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag), the water (W), and the acetone (Ac) was obtainedas Ag:W:Ac=80:20:100. The measurements of the properties showed that thesilver colloidal solution had a surface tension of 32 mN/m at 25° C., aviscosity of 14 mPa·s at 25° C., and a boiling point of 62° C. Anobservation of the dispersibility of the silver particles in the silvercolloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 5 nm. This result confirmed that theaddition of acetone after the concentrating operation did not vary theparticle size distribution.

In addition, the silver colloidal solution was applied onto a glasssubstrate. The formed film was dried and then observed with a scanningelectron microscope. As shown in FIG. 1, the observation confirmed thatthe formed film had a uniform structure composed of a number of silverparticles having a uniform particle diameter and shape.

Example 2

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 1 except that 12 grams of polyvinylpyrrolidone having a molecular weight of 25,000 was used as thedispersant.

The obtained silver colloidal solution was centrifuged at 20,000 G for20 minutes to remove impurities lighter than the silver particles. Thisremoving operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 12 nm.

Next, the obtained silver colloidal solution was concentrated with arotary evaporator to reduce the water content to 20 wt. %. Glycerin asthe water-soluble organic solvent was added to the solution. Thus, asilver colloidal solution whose dispersion medium was a mixed solutionof water and glycerin was produced.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag), the water (W), and the glycerin (Gl) was obtainedas Ag:W:Gl=80:20:200. The measurements of the properties showed that thesilver colloidal solution had a surface tension of 55 mN/m at 25° C., aviscosity of 850 mPa·s at 25° C., and a boiling point of 270° C. Anobservation of the dispersibility of the silver particles in the silvercolloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 12 nm. This result confirmed that theaddition of glycerin after the concentrating operation did not vary theparticle size distribution.

Example 3

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 1 except that the amount of the silvernitrate was changed to 48 grams, the amount of the polyvinyl pyrrolidone(molecular weight: 30,000) as the dispersant was changed to 24 grams,and 50 grams of citric acid was used as the reducing agent.

The obtained silver colloidal solution was centrifuged at 20,000 G for20 minutes to remove impurities lighter than the silver particles. Thisremoving operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 22 nm.

Next, the obtained silver colloidal solution was concentrated by heatingit at 60° C. to reduce the water content to 1 wt. %. Ethanol as thewater-soluble organic solvent was added to the solution. Thus, a silvercolloidal solution whose dispersion medium was a mixed solution of waterand ethanol was produced.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag), the water (W), and the ethanol (Et) was obtainedas Ag:W:Et=99:1:200. The measurements of the properties showed that thesilver colloidal solution had a surface tension of 24 mN/m at 25° C., aviscosity of 5 mPa·s at 25° C., and a boiling point of 82C. Anobservation of the dispersibility of the silver particles in the silvercolloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 22 nm. This result confirmed that theaddition of ethanol after the concentrating operation did not vary theparticle size distribution.

Example 4

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 1 except that the amount of the silvernitrate was changed to 48 grams, the amount of the polyvinyl pyrrolidone(molecular weight: 30,000) as the dispersant was changed to 30 grams,and the amount of the ethylene glycol as the reducing agent was changedto 250 grams.

The obtained silver colloidal solution was electrodialyzed using anultrafiltration membrane to remove impurities. The solution was rinsedwith pure water. The particle size distribution of the silver particleswas measured using the above-described particle size analyzer. Theresult revealed that there was a sharp peak at a position of 5 nm.

Next, the obtained silver colloidal solution was concentrated by heatingit at 60° C. to reduce the water content to 5 wt. %. Ethylene glycolmonobutyl ether and glycerin both as the water-soluble organic solventwere added to the solution. Thus, a silver colloidal solution whosedispersion medium was a mixed solution of water, ethylene glycolmonobutyl ether, and glycerin was produced.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag), the water (W), the ethylene glycol monobutylether (EG), and the glycerin (Gl) was obtained asAg:W:EG:Gl=95:5:400:10. The measurements of the properties showed thatthe silver colloidal solution had a surface tension of 33 mN/m at 25°C., a viscosity of 18 mPa·s at 25° C., and a boiling point of 250° C. Anobservation of the dispersibility of the silver particles in the silvercolloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 5 nm. This result confirmed that theaddition of ethylene glycol monobutyl ether and glycerin after theconcentrating operation did not vary the particle size distribution.

Example 5

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 1 except that the amount of the silvernitrate was changed to three grams, two grams of polyacrylic acid havinga molecular weight of 5,000 was used as the dispersant, and eight gramsof citric acid was used as the reducing agent.

The obtained silver colloidal solution was centrifuged at 20,000 G for20 minutes to remove impurities lighter than the silver particles. Thisremoving operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 31 nm.

Next, the obtained silver colloidal solution was concentrated bycentrifuging it at 10,000 rpm and then removing the supernatant fluid toreduce the water content to 12 wt. %. Then, 2-propanol as thewater-soluble organic solvent was added to the solution. Thus, a silvercolloidal solution whose dispersion medium was a mixed solution of waterand 2-propanol was produced.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag), the water (W), and the 2-propanol (Pr) wasobtained as Ag:W:Pr=88:12:100. The measurements of the properties showedthat the silver colloidal solution had a surface tension of 35 mN/m at25° C., a viscosity of 10 mPa·s at 25° C., and a boiling point of 95C.An observation of the dispersibility of the silver particles in thesilver colloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 31 nm. This result confirmed that theaddition of 2-propanol after the concentrating operation did not varythe particle size distribution.

Example 6

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 1 except that the amount of the silvernitrate was changed to 12 grams, 30 grams of poval having a molecularweight of 2,000 was used as the dispersant, 26 grams of glucose was usedas the reducing agent, the reaction temperature was changed to 60° C.,and the reaction time was changed to 60 minutes.

The obtained silver colloidal solution was electrodialyzed using anultrafiltration membrane to remove impurities. The solution was rinsedwith pure water. The particle size distribution of the silver particleswas measured using the above-described particle size analyzer. Theresult revealed that there was a sharp peak at a position of 10 nm.

Next, the obtained silver colloidal solution was concentrated by heatingit at 60° C. to reduce the water content to 8 wt. %. Then,2-ethoxyethanol and 2-pyrrolidone both as the water-soluble organicsolvent were added to the solution. Thus, a silver colloidal solutionwhose dispersion medium was a mixed solution of water, 2-ethoxyethanol,and 2-pyrrolidone was produced.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag), the water (W), the 2-ethoxyethanol (EE), and the2-pyrrolidone (Py) was obtained as Ag:W:EE:Py=92:8:150:40. Themeasurements of the properties showed that the silver colloidal solutionhad a surface tension of 30 mN/m at 25° C., a viscosity of 19 mPa·s at25° C., and a boiling point of 190° C. An observation of thedispersibility of the silver particles in the silver colloidal solutionshowed that when the solution was maintained stationary for two months,no precipitation occurred, which means the result was excellent. Itsparticle size distribution was measured using the above-describedparticle size analyzer. The result showed that there was a sharp peak ata position of 10 nm. This result confirmed that the addition of2-ethoxyethanol and 2-pyrrolidone after the concentrating operation didnot vary the particle size distribution.

Comparative Example 1

The water-based silver colloidal solution that was obtained in Example 1and that had a sharp peak at a position of 5 nm was heated at 60° C. toevaporate the water and remove it completely. Thus, the silver particleswere separated from water completely. Acetone was added to the particlesto produce a silver colloidal solution whose dispersion medium wasacetone.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag) and the acetone (Ac) was obtained as Ag:Ac=15:85.The measurements of the properties showed that the silver colloidalsolution had a surface tension of 25 mN/m at 25° C., a viscosity of 1mPa·s at 25° C., and a boiling point of 58° C. An observation of thedispersibility of the silver particles in the silver colloidal solutionshowed that when the solution was maintained stationary for one day,precipitation of silver particles occurred, which means thedispersibility was unsatisfactory. The particle size distribution of thesilver particles was measured using the above-described particle sizeanalyzer. The result revealed that there was a peak at a position of 250nm. This result explained that the addition of acetone after thecomplete separation of the silver particles from the water caused silverparticles to aggregate, so that the particle size distribution variedgreatly.

Comparative Example 2

A silver colloidal solution whose dispersion medium was made of waterand toluene was produced through a method similar to that used inExample 1 except the following points. After the completion of thereaction, the water-based silver colloidal solution was centrifuged,rinsed, and subjected to the measurement of particle size distribution.The solution was concentrated by centrifuging it at 10,000 rpm and thenremoving the supernatant fluid to reduce the water content to 10 wt. %.Then, toluene, which is a water-insoluble organic solvent, was added tothe solution. However, it was impossible to obtain a uniform silvercolloidal solution, because the solution separated into two types ofliquids. The solution had a boiling point of 120° C.

Comparative Example 3

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 1 except that 25 grams of citric acidwas used as the reducing agent.

The obtained silver colloidal solution was centrifuged at 20,000 G for20 minutes to remove impurities lighter than the silver particles. Thisremoving operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 22 nm.

Next, the obtained silver colloidal solution was heated at 60° C. toevaporate the water and remove it completely. Thus, the silver particleswere separated from water completely. Then, 2-propanol was added to theparticles to produce a silver colloidal solution whose dispersion mediumwas 2-propanol.

In the silver colloidal solution, the composition ratio in weight of thesilver particles (Ag) and the 2-propanol (Pr) was obtained asAg:Pr=100:100. The measurements of the properties showed that the silvercolloidal solution had a surface tension of 28 mN/m at 25° C., aviscosity of 5 mPa·s at 25° C., and a boiling point of 88C. Anobservation of the dispersibility of the silver particles in the silvercolloidal solution showed that when the solution was maintainedstationary for one day, precipitation of silver particles occurred,which means the dispersibility was unsatisfactory. The particle sizedistribution of the silver particles was measured using theabove-described particle size analyzer. The result showed that there wasa peak at a position of 1,500 nm. This result explained that theaddition of 2-propanol after the complete separation of the silverparticles from the water caused silver particles to aggregate, so thatthe particle size distribution varied greatly.

(Gold Colloidal Solution)

Example 7

First, 40 grams of tetrachloroaurate (III) 4hydrate was dissolved in 200grams of pure water. Second, 12 grams of polyvinyl pyrrolidone(molecular weight: 25,000) as the dispersant was added to the solutionand dissolved in it completely. Third, 10 grams of apple acid as thereducing agent was added to the gold chloride solution. The solution wasstirred at a stirring rate of 1,000 rpm for 60 minutes at 25° C. tocause the chemical reaction to proceed. Thus, a water-based goldcolloidal solution having a reddish-purple plasmon absorption wasobtained.

Next, the obtained gold colloidal solution was centrifuged at 20,000 Gfor 20 minutes to remove impurities lighter than the gold particles.This removing operation was repeated. The solution was rinsed with purewater. The particle size distribution of the gold particles was measuredusing the above-described particle size analyzer. The result revealedthat there was a sharp peak at a position of 20 nm.

The obtained gold colloidal solution was concentrated by centrifuging itat 10,000 rpm and then removing the supernatant fluid to reduce thewater content to 15 wt. %. Dipropylene glycol as the water-solubleorganic solvent was added to the solution. Thus, a gold colloidalsolution whose dispersion medium was a mixed solution of water anddipropylene glycol was produced.

In the gold colloidal solution, the composition ratio in weight of thegold particles (Au), the water (W), and the dipropylene glycol (DPG) wasobtained as Au:W:DPG=85:15:200. The measurements of the propertiesshowed that the gold colloidal solution had a surface tension of 40 mN/mat 25° C., a viscosity of 85 mPa·s at 25° C., and a boiling point of210° C. An observation of the dispersibility of the gold particles inthe gold colloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 20 nm. This result confirmed that theaddition of dipropylene glycol after the concentrating operation did notvary the particle size distribution.

Example 8

The water-based gold colloidal solution that was obtained in Example 7and that had a sharp peak at a position of 20 nm was concentrated with arotary evaporator to reduce the water content to 25 wt. %. Ethyl acetateas the water-soluble organic solvent was added to the solution. Thus, agold colloidal solution whose dispersion medium was a mixed solution ofwater and ethyl acetate was produced.

In the gold colloidal solution, the composition ratio in weight of thegold particles (Au), the water (W), and the ethyl acetate (EAc) wasobtained as Au:W:EAc=75:25:200. The measurements of the propertiesshowed that the gold colloidal solution had a surface tension of 28 mN/mat 25° C., a viscosity of 2 mPa·s at 25° C., and a boiling point of 88°C. An observation of the dispersibility of the gold particles in thegold colloidal solution showed that when the solution was maintainedstationary for two months, no precipitation occurred, which means theresult was excellent. Its particle size distribution was measured usingthe above-described particle size analyzer. The result showed that therewas a sharp peak at a position of 20 nm. This result confirmed that theaddition of ethyl acetate after the concentrating operation did not varythe particle size distribution.

Comparative Example 4

The water-based gold colloidal solution that was obtained in Example 7and that had a sharp peak at a position of 20 nm was heated at 60° C. toevaporate the water and remove it completely. Thus, the gold particleswere separated from water completely. Ethanol was added to the particlesto produce a gold colloidal solution whose dispersion medium wasethanol.

In the gold colloidal solution, the composition ratio in weight of thegold particles (Au) and the ethanol (Et) was obtained as Au:Et=100:200.The measurements of the properties showed that the gold colloidalsolution had a surface tension of 23 mN/m at 25° C., a viscosity of 3mPa·s at 25° C., and a boiling point of 85° C.

An observation of the dispersibility of the gold particles in the goldcolloidal solution showed that when the solution was maintainedstationary for one day, precipitation of gold particles occurred, whichmeans the dispersibility was unsatisfactory. The particle sizedistribution of the gold particles was measured using theabove-described particle size analyzer. The result revealed that therewas a peak at a position of 350 nm. This result explained that theaddition of ethanol after the complete separation of the gold particlesfrom the water caused gold particles to aggregate, so that the particlesize distribution varied greatly.

(Platinum Colloidal Solution)

Example 9

First, 45 grams of hexachloroplatinate (IV) 6hydrate was dissolved in200 grams of pure water. Second, eight grams of polyacrylic acid(molecular weight: 5,000) as the dispersant was added to the solutionand dissolved in it completely. Third, 80 grams of ethanol as thereducing agent was added to the platinum chloride solution. The solutionwas stirred at a stirring rate of 1,000 rpm for 360 minutes at 50° C. tocause the chemical reaction to proceed. Thus, a water-based platinumcolloidal solution having a black color was obtained.

Next, the obtained platinum colloidal solution was electrodialyzed usingan ultrafiltration membrane to remove impurities. The solution wasrinsed with pure water. The particle size distribution of the platinumparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 3 nm.

The obtained platinum colloidal solution was concentrated by heating itat 60° C. to reduce the water content to 5 wt. %. Acetone and glycerinboth as the water-soluble organic solvent were added to the solution.Thus, a platinum colloidal solution whose dispersion medium was a mixedsolution of water, acetone, and glycerin was produced.

In the platinum colloidal solution, the composition ratio in weight ofthe platinum particles (Pt), the water (W), the acetone (Ac), and theglycerin (Gl) was obtained as Pt:W:Ac:Gl=95:5:80:10. The measurements ofthe properties showed that the platinum colloidal solution had a surfacetension of 30 mN/m at 25° C., a viscosity of 16 mPa·s at 25° C., and aboiling point of 81° C. An observation of the dispersibility of theplatinum particles in the platinum colloidal solution showed that whenthe solution was maintained stationary for two months, no precipitationoccurred, which means the result was excellent. Its particle sizedistribution was measured using the above-described particle sizeanalyzer. The result showed that there was a sharp peak at a position of3 nm. This result confirmed that the addition of acetone and glycerinafter the concentrating operation did not vary the particle sizedistribution.

Example 10

A water-based platinum colloidal solution was obtained through a methodsimilar to that used in Example 9 except that 35 grams of poval having amolecular weight of 2,000 was used as the dispersant, 100 grams ofethylene glycol was used as the reducing agent, the reaction temperaturewas changed to 80° C., and the reaction time was changed to 180 minutes.

The obtained platinum colloidal solution was centrifuged at 20,000 G for20 minutes to remove impurities lighter than the platinum particles.This removing operation was repeated. The solution was rinsed with purewater. The particle size distribution of the platinum particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 10 nm.

Next, the obtained platinum colloidal solution was concentrated using arotary evaporator to reduce the water content to 15 wt. %. Then,tripropylene glycol monomethyl ether as the water-soluble organicsolvent was added to the solution. Thus, a platinum colloidal solutionwhose dispersion medium was a mixed solution of water and tripropyleneglycol monomethyl ether was produced.

In the platinum colloidal solution, the composition ratio in weight ofthe platinum particles (Pt), the water (W), and the tripropylene glycolmonomethyl ether (TPG) was obtained as Pt:W:TPG=85:15:400. Themeasurements of the properties showed that the platinum colloidalsolution had a surface tension of 35 mN/m at 25° C., a viscosity of 10mPa·s at 25° C., and a boiling point of 220° C. An observation of thedispersibility of the platinum particles in the platinum colloidalsolution showed that when the solution was maintained stationary for twomonths, no precipitation occurred, which means the result was excellent.Its particle size distribution was measured using the above-describedparticle size analyzer. The result showed that there was a sharp peak ata position of 10 nm. This result confirmed that the addition oftripropylene glycol monomethyl ether after the concentrating operationdid not vary the particle size distribution.

Comparative Example 5

The water-based platinum colloidal solution that was obtained in Example10 and that had a sharp peak at a position of 10 nm was treated with arotary evaporator to evaporate the water and remove it completely. Thus,the platinum particles were separated from water completely. Then,tetradecane, which is a water-insoluble organic solvent, was added tothe particles to produce a platinum colloidal solution. However, it wasimpossible to obtain a uniform platinum colloidal solution, because thesolution separated into two types of liquids. The solution had a boilingpoint of 240° C.

(Inkjet-Use Silver Ink)

Example 11

A silver nitrate-ammonia solution was prepared by, first, dissolving 26grams of silver nitrate in 200 grams of pure water and, then, addingammonia water to the liquid to adjust its pH to 11.0. Next, 12 grams ofpolyvinyl pyrrolidone (molecular weight: 30,000) as the dispersant wasadded to the silver nitrate-ammonia solution and dissolved in it. Then,100 grams of ethylene glycol as the reducing agent was added to thesolution. The solution was stirred at a stirring rate of 1,000 rpm for180 minutes at 10° C. to cause the chemical reaction to proceed. Thus, awater-based silver colloidal solution having a yellow plasmon absorptionwas obtained.

Next, the obtained silver colloidal solution was centrifuged at 20,000 Gfor 20 minutes to remove impurities lighter than the silver particles.This removing operation was repeated. The solution was rinsed with purewater. The partide size distribution of the silver particles wasmeasured using a particle size analyzer that applied the laser Dopplermethod (the analyzer: made by Nikkiso Co., Ltd., the brand name:Microtrac UPA150EX). The result revealed that there was a sharp peak ata position of 5 nm.

The obtained silver colloidal solution was concentrated with a rotaryevaporator to reduce the water content to 20 wt. %. Ethylene glycolmonobutyl ether and glycerin both as the water-soluble organic solventwere added to the solution. Then, the solution was stirred with amagnetic stirrer until the added solvents were completely dispersed inthe solution. The solution was concentrated by heating it until theconcentration of the silver particles reached 30 wt. %. Thus, aninkjet-use silver ink whose dispersion medium was a mixed solution ofwater, ethylene glycol monobutyl ether, and glycerin was produced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the water (W), the ethylene glycol monobutylether (EGB), and the glycerin (Gl) was obtained asAg:W:EGB:Gl=30:5:60:5. The measurements of the properties showed thatthe inkjet-use silver ink had a surface tension of 33 mN/m at 25° C., aviscosity of 16 mPa·s at 25° C., and a boiling point of 220° C. Theparticle size distribution of the silver particles in the inkjet-usesilver ink was measured using the above-described particle sizeanalyzer. The result showed that there was a sharp peak at a position of5 nm. This result confirmed that the implementation of the productionprocess of the inkjet-use silver ink did not vary the particle sizedistribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-usesilver ink was printed on the surface of a glass substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 300°C. for 30 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as L/S=50 μm/50μm. The thickness of the wiring circuit was measured using a surfaceroughness tester (made by Tokyo Seimitsu Co., Ltd. based in Japan). Theresult was 0.3 μm. The wiring circuit had a resistivity of 2.2×10⁻⁶Ω·cm.

Before the baking, the printed figure on the glass substrate wasobserved with a scanning electron microscope. As shown in FIG. 2, theobservation confirmed that the printed figure had a uniform structurecomposed of a number of silver particles having a uniform particlediameter and shape. Similarly, the wiring circuit formed by baking theprinted figure was also observed with a scanning electron microscope. Asshown in FIG. 3, the observation confirmed that the film forming thewiring circuit had a structure in which a number of molten silverparticles were consolidated continuously and neatly.

Example 12

A silver nitrate-ammonia solution was prepared by, first, dissolving 26grams of silver nitrate in 300 grams of pure water and, then, addingammonia gas to the liquid to adjust its pH to 11.3. Next, 16 grams ofpolyacrylic acid (molecular weight: 5,000) as the dispersant was addedto the silver nitrate-ammonia solution and dissolved in it. Then, 22grams of ascorbic acid as the reducing agent was added to the solution.The solution was stirred at a stirring rate of 1,000 rpm for fiveminutes at 5° C. to cause the chemical reaction to proceed. Thus, awater-based silver colloidal solution having a yellow plasmon absorptionwas obtained.

Next, the obtained silver colloidal solution was electrodialyzed usingan ultrafiltration membrane to remove impurities. The solution wasrinsed with pure water. The particle size distribution of the silverparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 15 nm.

The obtained silver colloidal solution was concentrated by heating it at70° C. to reduce the water content to 5 wt. %. Then, 2-butoxyethanol andpolyethylene glycol (average molecular weight: 200) both as thewater-soluble organic solvent were added to the solution. The solutionwas stirred with a magnetic stirrer until the added solvents werecompletely dispersed in the solution. The solution was concentrated byheating it until the concentration of the silver particles reached 30wt. %. Thus, an inkjet-use silver ink whose dispersion medium was amixed solution of water, 2-butoxyethanol, and polyethylene glycol wasproduced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the water (W), the 2-butoxyethanol (BE), and thepolyethylene glycol (PEG) was obtained as Ag:W:BE:PEG=30:1:65:4. Themeasurements of the properties showed that the inkjet-use silver ink hada surface tension of 30 mN/m at 25° C., a viscosity of 15 mPa·s at 25°C., and a boiling point of 150° C. The particle size distribution of thesilver particles in the inkjet-use silver ink was measured using theabove-described particle size analyzer. The result showed that there wasa sharp peak at a position of 15 nm. This result confirmed that theimplementation of the production process of the inkjet-use silver inkdid not vary the particle size distribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-usesilver ink was printed on the surface of a glass substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 200°C. for 30 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as L/S=50 μm/50μm. The thickness of the wiring circuit was measured using a surfaceroughness tester (made by Tokyo Seimitsu Co., Ltd.). The result was 0.3μm. The wiring circuit had a resistivity of 2.2×10⁻⁶ Ω·cm.

Example 13

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 12 except that the amount of the silvernitrate was changed to four grams, two grams of polyethylene imine(molecular weight: 600) was used as the dispersant, four grams oftitanium trichloride was used as the reducing agent, the reactiontemperature was changed to 25° C., and the reaction time was changed to30 minutes.

Next, the obtained silver colloidal solution was electrodialyzed usingan ultrafiltration membrane to remove impurities. The solution wasrinsed with pure water. The particle size distribution of the silverparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 8 nm.

The obtained silver colloidal solution was concentrated using a rotaryevaporator to reduce the water content to 15 wt. %. Ethylene glycol asthe water-soluble organic solvent was added to the solution. Then, thesolution was stirred with a magnetic stirrer until the added solvent wascompletely dispersed in the solution. The solution was concentrated byheating it until the concentration of the silver particles reached 25wt. %. Thus, an inkjet-use silver ink whose dispersion medium was amixed solution of water and ethylene glycol was produced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver partides (Ag), the water (W), and the ethylene glycol (EG) wasobtained as Ag:W:EG=25:2:73. The measurements of the properties showedthat the inkjet-use silver ink had a surface tension of 45 mN/m at 25°C., a viscosity of 19 mPa·s at 25° C., and a boiling point of 185C. Theparticle size distribution of the silver particles in the inkjet-usesilver ink was measured using the above-described particle sizeanalyzer. The result showed that there was a sharp peak at a position of8 nm. This result confirmed that the implementation of the productionprocess of the inkjet-use silver ink did not vary the partide sizedistribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-usesilver ink was printed on the surface of a glass substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 400°C. for 15 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as L/S=100μm/100 μm. The thickness of the wiring circuit was measured using asurface roughness tester (made by Tokyo Seimitsu Co., Ltd.). The resultwas 0.2 μm. The wiring circuit had a resistivity of 1.8×10⁻⁶ Ω·cm.

Example 14

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 12 except that 12 grams of polyethyleneimine (molecular weight: 10,000) was used as the dispersant, 20 grams offructose was used as the reducing agent, the reaction temperature waschanged to 15° C., and the reaction time was changed to 120 minutes.

Next, the obtained silver colloidal solution was centrifuged at 20,000 Gfor 20 minutes to remove impurities lighter than the silver particles.This removing operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 30 nm.

The obtained silver colloidal solution was concentrated by heating it at70° C. to reduce the water content to 3 wt. %. Then, 2-ethoxyethanol andglycerin both as the water-soluble organic solvent were added to thesolution. The solution was stirred with a magnetic stirrer until theadded solvents were completely dispersed in the solution. The solutionwas concentrated by heating it until the concentration of the silverparticles reached 40 wt. %. Thus, an inkjet-use silver ink whosedispersion medium was a mixed solution of water, 2-ethoxyethanol, andglycerin was produced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the water (W), and the 2-ethoxyethanol (EE) andthe glycerin (Gl) was obtained as Ag:W:EE:Gl=40:1:55:4. The measurementsof the properties showed that the inkjet-use silver ink had a surfacetension of 32 mN/m at 25° C., a viscosity of 14 mPa·s at 25° C., and aboiling point of 200° C. The particle size distribution of the silverparticles in the inkjet-use silver ink was measured using theabove-described particle size analyzer. The result showed that there wasa sharp peak at a position of 30 nm. This result confirmed that theimplementation of the production process of the inkjet-use silver inkdid not vary the particle size distribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-usesilver ink was printed on the surface of a synthetic-resin substrateusing a piezoelectric-type inkjet printer. The printed figure was bakedat 180° C. for 60 minutes in the atmosphere. Thus, a wiring circuit wasformed that had a line width of L and a spacing of S expressed as L/S=30μm/30 μm. The thickness of the wiring circuit was measured using asurface roughness tester (made by Tokyo Seimitsu Co., Ltd.). The resultwas 0.5 μm. The wiring circuit had a resistivity of 4.0×10⁻⁶ Ω·cm.

Example 15

A silver nitrate-ammonia solution was prepared by, first, dissolving 52grams of silver nitrate in 200 grams of pure water and, then, addingammonia water to the liquid to adjust its pH to 12.0. Next, 25 grams ofpolyvinyl pyrrolidone (molecular weight: 30,000) as the dispersant wasadded to the silver nitrate-ammonia solution and dissolved in it. Then,100 grams of 2-propanol as the reducing agent was added to the solution.The solution was stirred at a stirring rate of 1,000 rpm for 60 minutesat 40° C. to cause the chemical reaction to proceed. Thus, a water-basedsilver colloidal solution having a yellow plasmon absorption wasobtained.

Next, the obtained silver colloidal solution was centrifuged at 20,000 Gfor 20 minutes to remove impurities lighter than the silver particles.This removing operation was repeated. The solution was rinsed with purewater. The particle size distribution of the silver particles wasmeasured using the above-described particle size analyzer. The resultrevealed that there was a sharp peak at a position of 5 nm.

The obtained silver colloidal solution was concentrated using a rotaryevaporator to reduce the water content to 22 wt. %. Then,2-ethoxyethanol and 2-pyrrolidone both as the water-soluble organicsolvent were added to the solution. The solution was stirred with amagnetic stirrer until the added solvents were completely dispersed inthe solution. The solution was concentrated by heating it until theconcentration of the silver particles reached 30 wt. %. Thus, aninkjet-use silver ink whose dispersion medium was a mixed solution ofwater, 2-ethoxyethanol, and 2-pyrrolidone was produced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the water (W), the 2-ethoxyethanol (EE), and the2-pyrrolidone (Py) was obtained as Ag:W:EE:Py=30:5:60:5. Themeasurements of the properties showed that the inkjet-use silver ink hada surface tension of 32 mN/m at 25° C., a viscosity of 8 mPa·s at 25°C., and a boiling point of 195° C. The particle size distribution of thesilver particles in the inkjet-use silver ink was measured using theabove-described particle size analyzer. The result showed that there wasa sharp peak at a position of 5 nm. This result confirmed that theimplementation of the production process of the inkjet-use silver inkdid not vary the particle size distribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-usesilver ink was printed on the surface of a synthetic-resin substrateusing a piezoelectric-type inkjet printer. The printed figure was bakedat 200° C. for 30 minutes in the atmosphere. Thus, a wiring circuit wasformed that had a line width of L and a spacing of S expressed as L/S=80μm/80 μm. The thickness of the wiring circuit was measured using asurface roughness tester (made by Tokyo Seimitsu Co., Ltd.). The resultwas 0.3 μm. The wiring circuit had a resistivity of 3.5×10⁻⁶ Ω·cm.

Example 16

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 15 except that the amount of the silvernitrate was changed to 26 grams, 20 grams of polyvinyl pyrrolidone(molecular weight: 25,000) was used as the dispersant, 10 grams ofglutamic acid was used as the reducing agent, the reaction temperaturewas changed to 25° C., and the reaction time was changed to 30 minutes.

Next, the obtained silver colloidal solution was electrodialyzed usingan ultrafiltration membrane to remove impurities. The solution wasrinsed with pure water. The particle size distribution of the silverparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 40 nm.

The obtained silver colloidal solution was concentrated by centrifugingit at 10,000 rpm and then removing the supernatant fluid to reduce thewater content to 10 wt. %. Ethanol and glycerin both as thewater-soluble organic solvent were added to the solution. The solutionwas stirred with a magnetic stirrer until the added solvents werecompletely dispersed in the solution. The solution was concentrated byheating it until the concentration of the silver particles reached 60wt. %. Thus, an inkjet-use silver ink whose dispersion medium was amixed solution of water, ethanol, and glycerin was produced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the water (W), the ethanol (Et), and the glycerin(Gl) was obtained as Ag:W:Et:Gl=60:5:34:1. The measurements of theproperties showed that the inkjet-use silver ink had a surface tensionof 30 mN/m at 25° C., a viscosity of 3 mPa·s at 25° C., and a boilingpoint of 100° C. The particle size distribution of the silver particlesin the inkjet-use silver ink was measured using the above-describedparticle size analyzer. The result showed that there was a sharp peak ata position of 40 nm. This result confirmed that the implementation ofthe production process of the inkjet-use silver ink did not vary theparticle size distribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a thermal-type inkjet printer. The ink-ejecting stability wassatisfactory without developing any faulty ejection such as the cloggingat the nozzle of the printer. In addition, the inkjet-use silver ink wasprinted on the surface of a glass substrate using a thermal-type inkjetprinter. The printed figure was baked at 450° C. for 30 minutes in theatmosphere. Thus, a wiring circuit was formed that had a line width of Land a spacing of S expressed as L/S=120 μm/120 μm. The thickness of thewiring circuit was measured using a surface roughness tester (made byTokyo Seimitsu Co., Ltd.). The result was 1 μm. The wiring circuit had aresistivity of 1.7×10⁻⁶ Ω·cm.

Example 17

A water-based silver colloidal solution was obtained through a methodsimilar to that used in Example 15 except that the amount of the silvernitrate was changed to 26 grams, five grams of polyvinyl pyrrolidone(molecular weight: 30,000) was used as the dispersant, 20 grams ofmyristic acid was used as the reducing agent, the reaction temperaturewas changed to 25° C., and the reaction time was changed to 120 minutes.

Next, the obtained silver colloidal solution was electrodialyzed usingan ultrafiltration membrane to remove impurities. The solution wasrinsed with pure water. The particle size distribution of the silverparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 15 nm.

The obtained silver colloidal solution was concentrated by centrifugingit at 10,000 rpm and then removing the supernatant fluid to reduce thewater content to 12 wt. %. Ethanol as the water-soluble organic solventwas added to the solution. The solution was stirred with a magneticstirrer until the added solvent was completely dispersed in thesolution. The solution was concentrated by heating it until theconcentration of the silver particles reached 25 wt. %. Thus, aninkjet-use silver ink whose dispersion medium was a mixed solution ofwater and ethanol was produced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the water (W), and the ethanol (Et) was obtainedas Ag:W:Et=25:2:73. The measurements of the properties showed that theinkjet-use silver ink had a surface tension of 24 mN/m at 25° C., aviscosity of 2 mPa·s at 25° C., and a boiling point of 82C. The particlesize distribution of the silver particles in the inkjet-use silver inkwas measured using the above-described particle size analyzer. Theresult showed that there was a sharp peak at a position of 15 nm. Thisresult confirmed that the implementation of the production process ofthe inkjet-use silver ink did not vary the particle size distribution.

The inkjet-use silver ink was used for a one-hour continuous printingusing a thermal-type inkjet printer. The ink-ejecting stability wassatisfactory without developing any faulty ejection such as the cloggingat the nozzle of the printer. In addition, the inkjet-use silver ink wasprinted on the surface of a synthetic-resin substrate using athermal-type inkjet printer. The printed figure was baked at 180° C. for60 minutes in the atmosphere. Thus, a wiring circuit was formed that hada line width of L and a spacing of S expressed as L/S=50 μm/50 μm. Thethickness of the wiring circuit was measured using a surface roughnesstester (made by Tokyo Seimitsu Co., Ltd.). The result was 0.2 μm. Thewiring circuit had a resistivity of 6.5×10⁻⁶ Ω·cm.

Comparative Example 6

The water-based silver colloidal solution that was obtained in Example12 and that had a sharp peak at a position of 15 nm was heated at 70° C.to evaporate the water and remove it completely. Thus, the silverparticles were separated from water completely. Then, 2-butoxyethanoland polyethylene glycol (average molecular weight: 200) were added tothe solution. The solution was stirred with a magnetic stirrer until theadded solvents were completely dispersed in the solution. The solutionwas concentrated by heating it until the concentration of the silverparticles reached 30 wt. %. Thus, an inkjet-use silver ink whosedispersion medium was 2-butoxyethanol and polyethylene glycol wasproduced.

In the inkjet-use silver ink, the composition ratio in weight of thesilver particles (Ag), the 2-butoxyethanol (BE), and the polyethyleneglycol (PEG) was obtained as Ag:BE:PEG=30:50:20. The measurements of theproperties showed that the inkjet-use silver ink had a surface tensionof 65 mN/m at 25° C., a viscosity of 32 mPa·s at 25° C., and a boilingpoint of 150° C. The particle size distribution of the silver particlesin the inkjet-use silver ink was measured using the above-describedparticle size analyzer. The result showed that there was a peak at aposition of 1,500 nm. This result explained that when the silverparticles were separated from water completely, aggregation of thesilver particles developed, so that the particle size distributionvaried greatly.

The inkjet-use silver ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas unsatisfactory because faulty ejection developed, such as theclogging at the nozzle of the printer. In addition, the inkjet-usesilver ink was printed on the surface of a glass substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 200°C. for 30 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as LIS=300μm/300 μm. The thickness of the wiring circuit was measured using asurface roughness tester (made by Tokyo Seimitsu Co., Ltd.). The resultwas 0.3 μm. The wiring circuit had a resistivity of 2.5×10⁻⁵ Ωcm.

Comparative Example 7

The water-based silver colloidal solution that was obtained in Example13 and that had a sharp peak at a position of 8 nm was concentratedusing a rotary evaporator to reduce the water content to 15 wt. %. Then,α-terpineol, which is a water-insoluble organic solvent, was added tothe solution. The solution was stirred with a magnetic stirrer. However,it was impossible to obtain a uniform inkjet-use silver ink, because thesolution separated into two types of liquids.

(Inkjet-Use Palladium Ink)

Example 18

First, 21 grams of palladium chloride (II) was dissolved in 300 grams ofpure water. Second, 20 grams of polyacrylic acid (molecular weight:5,000) as the dispersant was added to the solution and dissolved in itcompletely. Third, 80 grams of 2-propanol as the reducing agent wasadded to the palladium chloride solution. The solution was stirred at astirring rate of 1,000 rpm for 60 minutes at 40° C. to cause thechemical reaction to proceed. Thus, a water-based palladium colloidalsolution having a black color was obtained.

Next, the obtained palladium colloidal solution was centrifuged at20,000 G for 20 minutes to remove impurities lighter than the palladiumparticles. This removing operation was repeated. The solution was rinsedwith pure water. The particle size distribution of the palladiumparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 3 nm.

The obtained palladium colloidal solution was concentrated by heating itat 70° C. to reduce the water content to 10 wt. %. Then, 2-ethoxyethanoland glycerin both as the water-soluble organic solvent were added to thesolution. The solution was stirred with a magnetic stirrer until theadded solvents were completely dispersed in the solution. The solutionwas concentrated by heating it until the concentration of the palladiumparticles reached 25 wt. %. Thus, an inkjet-use palladium ink whosedispersion medium was a mixed solution of water, 2-ethoxyethanol, andglycerin was produced.

In the inkjet-use palladium ink, the composition ratio in weight of thepalladium particles (Pd), the water (W), the 2-ethoxyethanol (EE), andthe glycerin (Gl) was obtained as Pd:W:EE:Gl=25:2:70:3. The measurementsof the properties showed that the inkjet-use palladium ink had a surfacetension of 32 mN/m at 25° C., a viscosity of 12 mPa·s at 25° C., and aboiling point of 200° C. The particle size distribution of the palladiumparticles in the inkjet-use palladium ink was measured using theabove-described particle size analyzer. The result showed that there wasa sharp peak at a position of 3 nm. This result confirmed that theimplementation of the production process of the inkjet-use palladium inkdid not vary the particle size distribution.

The inkjet-use palladium ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-usepalladium ink was printed on the surface of a glass substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 250°C. for 30 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as L/S=100μm/100 μm. The thickness of the wiring circuit was measured using asurface roughness tester (made by Tokyo Seimitsu Co., Ltd.). The resultwas 0.2 μm. The wiring circuit had a resistivity of 1.8×10⁻⁵ Ω·cm.

Example 19

A water-based palladium colloidal solution was obtained through a methodsimilar to that used in Example 18 except that five grams ofpolyethylene imine (molecular weight: 1,800) was used as the dispersant,16 grams of titanium trichloride was used as the reducing agent, thereaction temperature was changed to 25° C., and the reaction time waschanged to 30 minutes.

Next, the obtained palladium colloidal solution was electrodialyzedusing an ultrafiltration membrane to remove impurities. The solution wasrinsed with pure water. The particle size distribution of the palladiumparticles was measured using the above-described particle size analyzer.The result revealed that there was a sharp peak at a position of 4 nm.

The obtained palladium colloidal solution was concentrated using arotary evaporator to reduce the water content to 22 wt. %. Ethanol,glycerin, and acetone all as the water-soluble organic solvent wereadded to the solution. The solution was stirred with a magnetic stirreruntil the added solvents were completely dispersed in the solution. Thesolution was concentrated by heating it until the concentration of thepalladium particles reached 60 wt. %. Thus, an inkjet-use palladium inkwhose dispersion medium was a mixed solution of water, ethanol,glycerin, and acetone was produced.

In the inkjet-use palladium ink, the composition ratio in weight of thepalladium particles (Pd), the water (W), the ethanol (Et), the glycerin(Gl), and acetone (Ac) was obtained as Pd:W:Et:Gl:Ac=60:10:10:5:15. Themeasurements of the properties showed that the inkjet-use palladium inkhad a surface tension of 30 mN/m at 25° C., a viscosity of 1.5 mPa·s at25° C., and a boiling point of 95° C. The particle size distribution ofthe palladium particles in the inkjet-use palladium ink was measuredusing the above-described particle size analyzer. The result showed thatthere was a sharp peak at a position of 4 nm. This result confirmed thatthe implementation of the production process of the inkjet-use palladiumink did not vary the particle size distribution.

The inkjet-use palladium ink was used for a one-hour continuous printingusing a thermal-type inkjet printer. The ink-ejecting stability wassatisfactory without developing any faulty ejection such as the cloggingat the nozzle of the printer. In addition, the inkjet-use palladium inkwas printed on the surface of a glass substrate using a thermal-typeinkjet printer. The printed figure was baked at 200° C. for 30 minutesin the atmosphere. Thus, a wiring circuit was formed that had a linewidth of L and a spacing of S expressed as L/S=200 μm/200 μm. Thethickness of the wiring circuit was measured using a surface roughnesstester (made by Tokyo Seimitsu Co., Ltd.). The result was 0.4 μm. Thewiring circuit had a resistivity of 3.0×10⁻⁵ Ω·cm.

Comparative Example 8

The water-based palladium colloidal solution that was obtained inExample 19 and that had a sharp peak at a position of 4 nm wasconcentrated using a rotary evaporator to reduce the water content to 22wt. %. Then, α-terpineol, which is a water-insoluble organic solvent,was added to the solution. The solution was stirred with a magneticstirrer. However, it was impossible to obtain a uniform inkjet-usepalladium ink, because the solution separated into two types of liquids.

(Inkjet-Use Gold Ink)

Example 20

First, 42 grams of tetrachloroaurate (III) 4hydrate was dissolved in 150grams of pure water. Second, 16 grams of polyvinyl alcohol (molecularweight: 22,000) as the dispersant was added to the solution anddissolved in it completely. Third, 22 grams of ascorbic acid as thereducing agent was added to the gold chloride solution. The solution wasstirred at a stirring rate of 1,000 rpm for 15 minutes at 5° C. to causethe chemical reaction to proceed. Thus, a water-based gold colloidalsolution having a reddish-purple plasmon absorption was obtained.

Next, the obtained gold colloidal solution was electrodialyzed using anultrafiltration membrane to remove impurities. The solution was rinsedwith pure water. The particle size distribution of the gold particleswas measured using the above-described particle size analyzer. Theresult revealed that there was a sharp peak at a position of 5 nm.

The obtained gold colloidal solution was concentrated using a rotaryevaporator to reduce the water content to 15 wt. %. Then,2-butoxyethanol and polyethylene glycol (average molecular weight: 200)both as the water-soluble organic solvent were added to the solution.The solution was stirred with a magnetic stirrer until the addedsolvents were completely dispersed in the solution. The solution wasconcentrated by heating it until the concentration of the gold particlesreached 25 wt. %. Thus, an inkjet-use gold ink whose dispersion mediumwas a mixed solution of water, 2-butoxyethanol and polyethylene glycolwas produced.

In the inkjet-use gold ink, the composition ratio in weight of the goldparticles (Au), the water (W), the 2-butoxyethanol (BE), and thepolyethylene glycol (PEG) was obtained as Au:W:BE:PEG=25:4:65:6. Themeasurements of the properties showed that the inkjet-use gold ink had asurface tension of 35 mN/m at 25° C., a viscosity of 18 mPa·s at 25° C.,and a boiling point of 200° C. The particle size distribution of thegold particles in the inkjet-use gold ink was measured using theabove-described particle size analyzer. The result showed that there wasa sharp peak at a position of 5 nm. This result confirmed that theimplementation of the production process of the inkjet-use gold ink didnot vary the particle size distribution.

The inkjet-use gold ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas satisfactory without developing any faulty ejection such as theclogging at the nozzle of the printer. In addition, the inkjet-use goldink was printed on the surface of a synthetic-resin substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 150°C. for 120 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as L/S=50 μm/50μm. The thickness of the wiring circuit was measured using a surfaceroughness tester (made by Tokyo Seimitsu Co., Ltd.). The result was 0.3μm. The wiring circuit had a resistivity of 7.5×10⁻⁶ Ω·cm.

Example 21

A water-based gold colloidal solution was obtained through a methodsimilar to that used in Example 20 except that 10 grams of polyvinylpyrrolidone (molecular weight: 30,000) was used as the dispersant, 80grams of 2-propanol was used as the reducing agent, the reactiontemperature was changed to 40° C., and the reaction time was changed to60 minutes.

Next, the obtained gold colloidal solution was centrifuged at 20,000 Gfor 20 minutes to remove impurities lighter than the gold particles.This removing operation was repeated. The solution was rinsed with purewater. The particle size distribution of the gold particles was measuredusing the above-described particle size analyzer. The result revealedthat there was a sharp peak at a position of 15 nm.

The obtained gold colloidal solution was concentrated by heating it at70° C. to reduce the water content to 3 wt. %. Methyl ethyl ketone asthe water-soluble organic solvent was added to the solution. Then, thesolution was stirred with a magnetic stirrer until the added solvent wascompletely dispersed in the solution. The solution was concentrated byheating it until the concentration of the gold particles reached 40 wt.%. Thus, an inkjet-use gold ink whose dispersion medium was a mixedsolution of water and methyl ethyl ketone was produced.

In the inkjet-use gold ink, the composition ratio in weight of the goldparticles (Au), the water (W), and the methyl ethyl ketone (MEK) wasobtained as Au:W:MEK=40:1:59. The measurements of the properties showedthat the inkjet-use gold ink had a surface tension of 29 mN/m at 25° C.,a viscosity of 2 mPa·s at 25° C., and a boiling point of 85° C. Theparticle size distribution of the gold particles in the inkjet-use goldink was measured using the above-described particle size analyzer. Theresult showed that there was a sharp peak at a position of 15 nm. Thisresult confirmed that the implementation of the production process ofthe inkjet-use gold ink did not vary the particle size distribution.

The inkjet-use gold ink was used for a one-hour continuous printingusing a thermal-type inkjet printer. The ink-ejecting stability wassatisfactory without developing any faulty ejection such as the cloggingat the nozzle of the printer. In addition, the inkjet-use gold ink wasprinted on the surface of a glass substrate using a thermal-type inkjetprinter. The printed figure was baked at 350° C. for 20 minutes in theatmosphere. Thus, a wiring circuit was formed that had a line width of Land a spacing of S expressed as L/S=100 μm/100 μm. The thickness of thewiring circuit was measured using a surface roughness tester (made byTokyo Seimitsu Co., Ltd.). The result was 0.5 μm. The wiring circuit hada resistivity of 3.0×10⁻⁶ Ω cm.

Comparative Example 9

The water-based gold colloidal solution that was obtained in Example 20and that had a sharp peak at a position of 5 nm was treated with arotary evaporator to evaporate the water and remove it completely. Thus,the gold particles were separated from water completely. Then,2-butoxyethanol and polyethylene glycol (average molecular weight: 200)were added to the solution. The solution was stirred with a magneticstirrer until the added solvents were completely dispersed in thesolution. The solution was concentrated by heating it until theconcentration of the gold particles reached 10 wt. %. Thus, aninkjet-use gold ink whose dispersion medium was 2-butoxyethanol andpolyethylene glycol was produced.

In the inkjet-use gold ink, the composition ratio in weight of the goldparticles (Au), the 2-butoxyethanol (BE), and the polyethylene glycol(PEG) was obtained as Au:BE:PEG=10:85:5. The measurements of theproperties showed that the inkjet-use gold ink had a surface tension of65 mN/m at 25° C., a viscosity of 18 mPa·s at 25° C., and a boilingpoint of 200° C. The particle size distribution of the gold particles inthe inkjet-use gold ink was measured using the above-described particlesize analyzer. The result showed that there was a peak at a position of600 nm. This result explained that when the gold particles wereseparated from water completely, aggregation of the gold particlesdeveloped, so that the particle size distribution varied greatly.

The inkjet-use gold ink was used for a one-hour continuous printingusing a piezoelectric-type inkjet printer. The ink-ejecting stabilitywas unsatisfactory because faulty ejection developed, such as theclogging at the nozzle of the printer. In addition, the inkjet-use goldink was printed on the surface of a glass substrate using apiezoelectric-type inkjet printer. The printed figure was baked at 200°C. for 30 minutes in the atmosphere. Thus, a wiring circuit was formedthat had a line width of L and a spacing of S expressed as L/S=400μm/400 μm. The thickness of the wiring circuit was measured using asurface roughness tester (made by Tokyo Seimitsu Co., Ltd.). The resultwas 0.1 μm. The wiring circuit had a resistivity of 4.0×10⁻⁴ Ω·cm.

1. A metallic colloidal solution comprising: (a) metallic particlesthat: (a1) are deposited by reducing metallic ions in water; and (a2)have a primary-particle diameter of at most 200 nm; (b) a dispersanthaving a molecular weight of 200 to 30,000; and (c) a dispersion mediummade of a mixed solvent of water and a water-soluble organic solvent. 2.A metallic colloidal solution as defined by claim 1, the metalliccolloidal solution being produced: (a) by using as a starting material awater-based metallic colloidal solution obtained by depositing metallicparticles through the reduction of metallic ions in water; and (b)without undergoing a process that completely separates the metallicparticles from water.
 3. A metallic colloidal solution as defined byclaim 1, wherein the metallic particles are particles made of a metalselected from the group consisting of nickel, copper, silver, gold,platinum, palladium, and an alloy of these metals.
 4. A metalliccolloidal solution as defined by claim 1, wherein the content of themetallic particles is 0.1 to 90 wt. %.
 5. A metallic colloidal solutionas defined by claim 1, wherein the dispersant is an organic compoundthat does not contain any of sulfur, phosphorus, boron, and a halogenatom.
 6. A metallic colloidal solution as defined by claim 1, whereinthe amount of the dispersant is 2 to 30 weight parts per 100 weightparts of the metallic particles.
 7. A metallic colloidal solution asdefined by claim 1, wherein the water-soluble organic solvent is made ofat least one material selected from the group consisting of alcohol,ketone, glycol ether, and a water-soluble nitrogen-containing organiccompound.
 8. An inkjet-use metallic ink incorporating a metalliccolloidal solution comprising: (a) metallic particles that: (a1) aredeposited by reducing metallic ions in water; and (a2) have aprimary-particle diameter of at most 200 nm; (b) a dispersant having amolecular weight of 200 to 30,000; and (c) a dispersion medium made of amixed solvent of water and a water-soluble organic solvent.
 9. Aninkjet-use metallic ink as defined by claim 8, the inkjet-use metallicink having a surface tension of 20 to 60 mN/m at 25° C. and a viscosityof 0.5 to 40 mPa·s at 25° C.
 10. An inkjet-use metallic ink as definedby claim 8, wherein the metallic colloidal solution is produced: (a) byusing as a starting material a water-based metallic colloidal solutionobtained by depositing metallic particles through the reduction ofmetallic ions in water; and (b) without undergoing a process thatcompletely separates the metallic particles from water.
 11. Aninkjet-use metallic ink as defined by claim 8, wherein the water-solubleorganic solvent is made of at least one material selected from the groupconsisting of alcohol, ketone, glycol ether, and a water-solublenitrogen-containing organic compound.